Modulating system



June 30, 19.42,.

E. B. FERRELL MODULATING SYSTEM Original Filed June 9,1934 '2 Sheets-Sheet 1 FIG! I HIGH F RE GUEIVC I AME INVENTOR BBL FERRELL ATTORNEY June 30, 1942. E B. FER R ELL 2,233,275

7 MbDULATING SYSTEM I 2 Sheets-sheet 2 Original Filed Jung 9, 1934 FIG. 4

' llnilllllll INVENTOR E. B. FERRELL A ATTORNEY as much as may be desired.

Patented June so, 1942 c 2,288,275

UNITED STATES PATENT OFFICE MODULATING SYSTEM Enoch B. Ferrell, Oakhnrst, N. 3., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Original application June 9, 1934, Serial No. 729,735, now Patent No. 2,159,020, dated May 23, 1939. Divided and this application June 'o, 1938, Serial No. 211,548

19 Claims. (Cl. 179-1715) This invention relates to modulating systems directed particularly ,to those embodiments of and more particularly to means for regulating the invention that employ a self-detecting moduthe transmission characteristics of such systems. lator or amplifier to effect the detection required The invention has as its main object to reduce for the reduction of distortion and-noise, thereby wave distortion in a modulator, particularly.eneliminating a separate detector for that purvelope distortion of a modulated wave. pose.

Another object is to remove from an alternat- The invention will be more fully understood ing current or carrier wave any undesired modufrom-the following detailed description and from lation such as noise efi'ects and slow amplitude the accompanying drawings of which: variations. Fig. 1 shows a radio transmitting system re- Noise and modulation effects have been sucferred to in explaining the invention; cessfully controlled in amplifiers by means of Fig. 2 shows a radio transmittingsystem emnegative feedback or degeneration. The underbodying the invention: lying principles involved in this method of con- Fig. 3 shows an arrangement of the invention trol are disclosed by H; s.- Black in an article 1 as adapted to a screen grid amplifier; and

entitled Stabilized Feedback Amplifiers, pub- Fig. 4 shows a transmitting system incorporat- I lished in the January 1934 issue of the Bell ing initial noise reduction in a carrier supply System Technical Journal, on pages 118 and in amplifier.

a copending application, Serial No. 606,871, filed In order -to facilitate understanding of the April 22, 1932, Patent No. 2,102,671 December 21, species of the invention claimed herein, there is 1937, and assigned to the same assignee as the first shown in Fig. 1, a system employing a sepapresent application. In a modulator, however, rate detector for use in' reducing distortion and the problem of noise and distortion reduction is noise in order to emphasize the role of the desomewhat complicated by the necessary fretecting function.

quency changes which occur; In Fig. 1, a balanced high frequency generat- In accordance with the invention, detection is ing system is shown connected to a transmitemployed to derive a. detected signal from a ting antenna system 2 through ahigh frequency modulated carrier wave. In the absence of feedamplifier 3. A signal source 4 is associated with back in the. system the detected signal varies the output circuit of the generating system by somewhat from the original signal in its wave means of a transformer 5, a bridge network 6 form according to the amount of noise and disand a low frequency amplifier I. A receiving tortion introduced by the modulator. To reantenna 8 is provided with a rectifying detector duce this distortionthe detected signal is fed 9, the outputof the detector being connected back degeneratively upon the signal input of the through. a filter I0 and transformer H to the modulator. By making the fed-back component 85 bridge 6. Tra formers 5 and I l are placed in large compared with the original signal input the opposite diagonals of the bridge, with the and controlling its amplitude, the envelope disamplifier 1 across one of the ratio arms. Detortion of the modulated wave may be reduced tector 9 may be a diode vacuum tube or a copperoxide rectifier or other rectifying device exhibit- Preferably a rectifier having a linear char- 40 ingalinear relationship between its input voltage acteristic is used as the means for detecting and output current.

the signal from the modulated wave since such a In the operation of the system shown in Fig. rectifier produces a faithful copy of the en- 1, the high frequency output of the generator! velope of the modulated wave and introduces no is modulated by amplified signals from source 4 additional signal distortion by itself. transmitted "through .the' transformer 5, the

In a modified form of the invention the prinbridge 6. and the amplifier 1. The modulated ciple of degenerative feedback is used to remove 1 high frequency wave is amplified in the amplifier noise components and other undesired modu- '3 and radiated from the antenna-2. The antenlations from a carrier wave, thus providing a na 8 picks up energyfrom the radiated wave pure sinusoidal wave of constant amplitude for detector 9, which feeds the detected wave which may be supplied to a modulating amplifier back in reversed phase upon the input circuit of for signal modulation. I the low frequency amplifier I. The feedback This application is a division of my copendoperates to diminish noise effects and distortion ing application Serial No. 729,735, filed June 9, in the output wave from the system in accord- 1934, Patent 2,159,020, May 23, 1939, and is ance with the principles describedinthe hereinbefore mentioned paper by H. S. Black. The system diminishes noise and distortion originate ing in any portion of the loop comprising amplifier 1, generator I, amplifier I, detector 8, and their coupling arrangements. The greatest reduction of disturbing effects will occur when the feedback is as strongas possible and has a phase angle close to 180 degrees. Strong reversed feed back results,'of course, in reduced output, but the modulation can be restored to its former strength by increasing the signal input. The phase requirement can generally be met by reversing one winding of the transformer l I, because with high grade amplifiers the phase shift in any stage will be either zero or 180 degrees. If the total phase shift around the loop is 180 degrees or an odd multiple thereof, the transformer may not be needed.-

The general principles of reversed feedback operation are set forth in. Blacks article above cited. In the application of the principles to specific circuits it is well to note two definite limitations. One is that the system will not compensate for distortion which comes from interruption of the pace current of a vacuum tube.

Neither will it compensate for distortion due to exceeding the saturation voltage of a tube. The other limitation is imposed by the tendency to sing or set up self-sustaining oscillations.

Nevertheless Ihave found by experiment that a substantial reduction of noise and distortion can my invention.

The bridge network 6 serves to isolate the feedback circuit from the signal supply circuit, makingthese circuits conjugate to each other and capable of independent adjustment. The filter i .is employed to prevent high frequency feed back.

Fig. 2 shows the application of the invention to a modulating amplifier of class B or class C; that the figure that an increase of plate current through coil 18 makes the grid more negative, while a decrease of plate current makes the grid less negative, these variations in the grid voltage being opposite in phase to the signal variations which cause the respective changes in plate current. This'arrangement does not require a separate detector since the amplifier performs the detecting function. The system operates to diminish noise and distortion arising in the modulating amplifier or in the carrier generator.

" Fig. 3 shows an arrangement where the reversed feedback is from the plate circuit to the screen circuit of a screen grid tube. The carrier source I! is connected to the antenna 2 through a screen grid tube 20. The tube is provided with a large biasing battery 2| toeifect class B or C operation. The signal source 4 is associated with the antenna 2.

be secured in radio transmitters by the use ofis, to an amplifier operated with suilicient negative grid bias to cut oil or nearly cut oil. the plate current in the absence of impressed alternating current excitation. In the figure a carrier generator i2 is connected to the antenna 2 through a modulating amplifier iii. The signal source 4 is coupled to the input circuit of the amplifier it .by means of the bridge 6 and the transformer 5.

In the opposite bridge diagonal to the transformer 5 isinserted a low frequencycoil it which is part of the plate circuit of the tube IS. The grid circuit of the tube i3 includes one of the ratio arms of'the bridge. The plate supply battery is shown at i9.

In the operation of the system of Fig. 2, the carrier wave isimpressed upon the grid circuit of the tube I! along with the signal wave from the bridge 6. The plate current from the bat tery l9 passes through the bridge 6 providing a strong negative grid bias for the tube. fixed by the potential dro -in'thei ratio arm included in the grid circuit. Due to the strong bias theplate current fiows in pulses when the carrier and signal waves are impressed; In the presence of the carrier without signals the pulses are uniform, but when the signals are applied the pulses vary in strength according to the signals. Consequently the plate current contains a component of low frequency, which is substantially a faithful copy of theenvelope of the modulated wave in a properly adjusted modulator. The low fre'- quency wave traverses coil I8, thus feeding back uponthe grid of tube l3. This feedback is inherentlya reversed one as it will be evident from the screen electrode of tube 20 by means of the transformer 5. Coupling between the plate and screen circuits at low frequencies is provided by the transformer l I.

In the operation of the system of Fig. 3, the

carrier is impressed upon the rid and the signal upon the screenof the tube II. The modulated wave generated in the plate circuit is radiated by The low frequency component of the plate current generated by class B or C operation is fedback upon the screen by means of the transformer II which is so poled that an increase of plate current results in a decrease in the positive bias upon the screen and a decrease of plate current results in an increase of screen bias, which conditions evidently constitute reversed feedback. y 1

Fig. 4 illustrates a reversed feedback system in which the wave fed back is a component of the plate current in a class B power amplifier. The carrier source I2 is connected to the antenna 2 through a carrier supply amplifier 30, modulating amplifier l3 and power amplifier 11. The carrier amplifier 30 is provided with a choke coil II which is common to the plate and grid circuits of the amplifier. The signal source 4 is coupled to the grid of the tube It through the transformer 5 and the bridge 6. The plate circuit of the tube It is coupled to the grid circuit of thetube l3 by means of a resistance-l2. A

transmitted in reversed phase to the grid circuit v by'coil '3l, producing a compensatory variation in the gain of the amplifier. That is, an increase of plate current causes the grid potential to become more negative, decreasing the gain, and a decrease/ of plate current makes the grid become less negative, increasing the gain. The controlled carrier wave is impressed upon the grid of tube l3 together with the signalwave from tube ll,

1 the resulting modulated wave appearing in the plate circuit of tube It. The modulated wave'is amplified in tube I], the amplified wave being radiated by the antenna 2. The low frequency component of the plate current of tube I1 is fed back upon the grid of tube 54 by means of choke coil It. The system diminishes noise and distortion arising in amplifiers l3, l4 and I1 and also noise arising in generator l2 and tube 30.

What is claimed is:

1. A modulating system comprising a vacuum tube having a cathode, a grid and a plate, a grid circuit and a plate circuit therefor, a carrier source coupled to the grid circuit, a; bridge network, a signal source included in one diagonal of the bridge, and an impedance selective to low frequency included both in the other diagonal of the bridge and in the plate circuit of the tube, the grid circuit of the tube including a ratio arm of the bridge.

2. The method of correcting distortion nor-v mally produced in a modulation circuit comprising an electron discharge tube having input electrodes connected with a source of carrier waves and with a source of modulating potentials and having output electrodes connected through a modulation frequency impedor to produce in said impedor correcting potentials of modulation potential frequency which includes the steps of, impressing correcting potentials from said impedor on said. input circuit in phase opposition with respect to the modulating potentials im-' pressed thereon from said modulation potential source, and adjusting the amplitude of said correcting potentials to reduce distortion in said tube.

3. A' modulating system with distortion correc-' tion, comprising a carrier wave source, a signal wave source, a self-detecting modulator of a type adapted to generate a pulsating output current analyzable into a modulated carrier wave component and a wave component simulating the cathode circuit and at least one grid-cathode circuit therefor, means for setting u carrier claim 5 inwhich the modulation frequency imwaves in the anode-cathode circuit of said modulator under the control of said carrier wave source, means for impressing waves from said signal wave source upon a grid-cathode circuit of said modulator to generate said pulsating output current in said tube, decoupling means between said signal wave source and said anodecathode circuit, means connected to said modulator for selecting and utilizing said modulated carrier wave component of the output current.

means for selecting the said simulated signal wave componentv of the output current and means for impressing a potential variation cor-.

responding thereto upon the said grid-cathode circuit for distortion correction purposes.

4; A modulatin system comprising a discharge tube having a cathode, an anode and a grid. an anode-cathode circuit and a grid-cathode circuit therefor, a carrier source, means to set up carrier waves in the anode-cathode circuit under the control of said carrier source. a source of modulating potentials coupled to the gridcathode circuit, decoupling means between said source of modulating potentials and said anode cathode circuit, an impedance device having one terminal connected to the cathode and the other terminal connected both to the grid and to the anode and means for producing'in said impedtil tube having a cathode, an anode and a grid, an anode-cathode circuit and a grid-cathode circuit therefor, a carrier source, means to set up carrier waves in the anode-cathode circuit under the control'of said carrier source, a source of modulating potentials coupled to the grid-cathode 5 by there are produced in said modulation irequency impeder distortion correcting potentials 'of modulation potential frequency simulating the original modulating potentials with superposed distortion from the modulating process and whereby the said distortion correcting potentials \are inherently in phase opposition with respect to the modulating potentials impressed upon the grid-cathode circuit from said modulating potential source. I v

6. A modulating system in accordance with peder comprises an inductance.

7..A modulating system in accordance with claim 5 in which the modulation frequency impeder comprises a resistor shunted by a condenser.

8. 'A modulating system comprising a discharge tube having a cathode, an anode and a grid, anode-cathode and grid-cathode circuits there for, a carrier source, means to set up. carrier waves in the anode-cathode circuit under the control of said carrier source, a signal sourcecounpled to the grid-cathode circuit and an inductance having substantial reactance in the frequencyrange comprising the waves from said signal source and having one terminal connected to the cathode and the other terminal connected to the grid and to the. anode to selectively transfer between saidanode-cathode and grid-cathode circuits, waves in the said'frequency range.

9. A modulating system comprising a discharge tube having a cathode, an anode and a grid,

anode-cathode and grid-cathode circuits there-v for, a carrier source, means to set up carrier waves in the anode-cathode circuit under the control of said carrier source, a source of modulating potentials coupled to the grid-cathode circuit, decoupling means between said source of modulating potentials and said anode-cathode circuit, an impedance device having one terminal connected to the cathode and the other terminal c nnected both to the grid and to the anode, and

said distortion correcting potentials are inherently in phase opposition with respect to the modulating potentials impressed upon the grid-cathode circuit from said modulating potential 10. A modulating system withdistortion correction, comprising a discharge tube having a cathode, an anode and a grid, anode-cathode and grid-cathode circuits therefor, a carrier source, means to set up carrier waves in the anode-oath ode circuit of said tube under the control of said carrier source, a source of modulating potentials coupled with the grid-cathode circuit, decoupling means between said source of modulating potentials and said anode-cathode circuit, a modulation frequency impeder having one terminal connected to the cathode and the other terminal connected both to the grid and to the anode and means to substantially suppress the space current of the tube in the absence of impressed alternating current excitations, whereby there are developed in the anode-cathode circuit including said modulation frequency impeder distortion corrective currents of modulating potential frequency.

11. A modulating system comprising at least one high frequency amplifier-detector tube, grid and-plate circuits therefoig'a carrier source, a

utilization circuit for waves of carrier frequency in the plate circuit of said tube. agsignal source, means to combine carrier waves and signal waves from said respective sources to produce a modu-' lated carrier wave in the plate circuit of said amplifying tube, means to impress upon a grid of said amplifying tube suflicient negative biasing potential to substantially cut oil the space current of the tube in the absence of impressed alternating current excitation, whereby signal waves cause self-detected signal frequency variations in the space current of said amplifying .tube and means to feed said signal frequency space current variations into a grid circuit of the circuit, including input electrodes, said electrodes being connected with a source of carrier waves and with a source of modulating potentials and having output electrodes connected through a modulation frequency impeder toproduce in said impeder correctingpotentials of modulating potential frequency, which includes the steps of impressing modulating potentials upon said input circuit from said modulation potential source and impressing correcting potentials from said impeder on said input circuit in phase opposition with respect to the modulating potentials impressed thereon from said modulation potential source.

15. A modulation system comprising a source of carrier waves, a source of modulating potentials, a modulation frequency impeder, an electron discharge tube having input electrodes connected with said source of carrier waves and with system previously traversed by signal waves and in such phase as to effect substantial phase opposition between said signal waves and said signal frequency space current variations in said grid circuit.

12. A modulating system comprising a self-detecting amplifying" tube, grid and, plate circuits therefor, a carrier source, a utilization circuit for waves of carrier frequency in the plate circuit of said tube, a signal source, means to combine carsaid source of modulating potentials, and having,

output electrodes connected through said modulation frequency impeder, means to substantially suppress the space current of said tube in the absence of impressed alternating current excitations, whereby there are produced in said modulation frequency impeder correcting potentials of modulation potential frequency, and means for impressing correcting potentials from said impeder on said input electrodes in phase opposition with respect to the modulating potentials impressed thereon from said modulating potential source.

1 16. In a modulation system, a source of carrier wave oscillations, a source of modulating potentials, an electron discharge tube having a control grid, a cathode and an anode, a circuit cou pling said source of carrier wave'oscillations to said cathode and control grid, a circuit coupling rier waves and signal waves from said respective sources to produce a modulated carrier wave in' the plate circuit of said amplifying tube together with self-detected signal frequency variations in the space current of said tube, and means to feed said signal frequency space current variations into a grid circuit of the system previously prising a plurality of space discharge tubes connected in cascade, grid and plate circuits for said tubes, a utilization circuit for waves of carrier frequency in the plate circuit of one of said tubes, a carrier source, a signal source, means to modulate carrier waves and signal waves from said respective sources and thereby to produce a modulated carrier wave in the aforesaid plate circuit, means including a self-detecting amplifying tube whereby the application of signal waves to said system produces signal frequency variations in the space current associated with the aforesaid plate circuit and means to feed said signal frequency space current variations through a portion of the circuit of a preceding tube including a point'traversed by signal waves in substantial phase opposition to the said signal waves.

14. The method of correcting distortion normally producediin a modulation circuit compristrol grid and cathode of said tube to impress modulating potentials on said control grid and cathode to modulate in said'tube said oscillations substantially in accordance with said modulation potentials, said last circuit including an impedance and means to impress upon said control grid suflicient negative biasing potential to substantially cut off the space current of the tube in the absence of impressed alternating current excitation and means for correcting distortion inherently accomplished during modulation of said carrier wave oscillations by said modulating potentials including a source of direct curi rent potential and said impedance connected be- ,tween the anode and cathode of said tube, and

means for settingup in said impedance potentials characteristic of the modulation frequency flowing in said last-mentioned circuit which opv pose said modulating potentials.

ing an electron discharge tube having an input cathode of said device to said source of 0scilla-- tions, a transformer having its primary winding coupled to said sourceof modulating potentials, said transformer having a secondary winding in 'which said modulating potentials are induced, a

series circuit between the control grid and cathode, said series circuit including said secondary winding and a resistance and means to impress upon saidcontrol grid suflicient negative biasing potential to substantially cut ofi the space 'current of the tube in the'absence of impressed alternating current excitation, said last circuit being for applying said modulating potentials to said control grid and cathode to modulate the said oscillations substantially in accordance with said modulating potentials, an alternating current circuit connected with the anode and cathode of said device and meansjfor compensating for distortion inherently accomplished during said modulation including a circuit including a source of direct current potential. connecting the anode to the cathode of said device, and means in saids last-named circuit for producing potentials in said resistance of a value characteristic of the currents in said circuit which oppose said modulating potentials induced in said secondary winding.

18. A modulating system with distortion cor type adapted to generate a pulsating output current analyzable into amodulated carrier wave component and a wave component simulating the signal wave with superposed distortion from the modulating process, said modulator comprising an electron discharge tube with a cathode, an anode, a control grid and a screen grid, an anode-cathode circuit and a screen grid-cathode circuit therefor, means for setting up carrier waves in the anode-cathode circuit of said modulator under the control of said carrier wave rection, comprising a carrier wave source, a signal wave source, a self-detecting modulator of a source, means for impressing waves from said signal wave source upon said screen grid-cathode lating amplifier tube, a, combined power amplifier and detector tube, a low frequency amplifier tube, grid and plate circuits for each of said tubes, means for setting up carrier waves in the plate circuit of said modulating amplifier tube, a bridge network including an arm coupled to the grid circuit of said low frequency amplifier tube, a signal source included in one diagonal of said bridge network, an impedance element included both in the plate circuit of said poweramplifier and-detector tube and in the other diagonal of said bridge network and a second impedance element coupling the grid circuit of said modulating amplifier tube and the plate circuit of said low frequency amplifier tube.

ENOCHB. FERRELL. 

